Ball and shaft of joint prothesis

ABSTRACT

A prosthesis for replacement of a ball and socket joint in the human body. The prosthetic components comprise a plurality of separate parts which may be inserted through a hole in the femur of the patient and assembled and attached to the prepared acetabulum of the hip bone to form a cup-shaped first shell in a minimally invasive surgical procedure. A cup may be passed through the hole in the femur and attached to the first shell to form a socket portion of the joint. The prosthesis may comprise a shaft having a first end with a ball formed thereon that is inserted in the hole through the femur so that the ball engages the cup for movement therein.

RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No. 10/799,192, filed Mar. 12, 2004, which is a division of application Ser. No. 09/961,662, filed Sep. 24, 2001, now U.S. Pat. No. 6,755,865, the specification and drawings of application Ser. Nos. 10/799,192 and 09/961,662 are incorporated herein their entirety.

FIELD OF THE INVENTION

The field relates to devices for joint replacement, especially hip and shoulder joint prosthetics and procedures.

BACKGROUND OF THE INVENTION

Arthroplasty, the restoration of normal joint motion, is frequently done by the insertion of a prosthetic joint replacement. Implant technology has improved over the last number of years and provides solutions to problems caused by injury, arthritis and other joint diseases. Frequently, the damage is sufficiently severe to require a total joint replacement. The prior art discloses numerous designs for total hip joint prosthetic devices.

Total hip joint replacements require interactive prosthetic femoral and acetabular components to emulate the ball-socket mechanism of a natural hip joint. When the supporting structure is weakened, particularly the femoral head and neck, a prosthetic femoral component with an extended shaft is implanted within the medullary cavity of the femur. Examples of this type total hip replacement prosthetic device are disclosed in U.S. Pat. No. 6,093,208 issued to Enrico Tian and U.S. Pat. No. 5,807,407 issued to England, et al. Many surgeons take this route, even when the underlying bone structure of the femoral head and neck is strong, under the theory that implantation of the shaft within the medullary cavity of the femur is required to obtain the necessary support for the prosthetic femoral head, as the femoral implant is under high stresses that can cause failure of “surface replacement” devices. Such failures frequently occur early in the patient's recovery, before the bonding of the bone to the metal surfaces of the prosthetic implant has occurred. However, the insertion of the prosthetic device with a long femoral shaft requires the resection of the femoral head and neck to obtain access to the longitudinal cavity within the femur. Such surgery is very stressful to the patient and increases the risk of infection. If the device fails, any further implantation of prosthetic devices becomes exceedingly difficult, as the supporting bone structure has already been appreciably reduced.

U.S. Pat. No. 5, 800,558 to Gerald A. LaHaise, Sr., U.S. Pat. No. 5,133,764 to Pappas et al., and U. S. Pat. No. 4,846,841 to Indong Oh, disclose the “surface replacement” technique of a total replacement of a hip joint. “Surface replacement” is aimed at primarily providing replacement of the joint surfaces while preserving as much of the supporting bone structure as possible and preserving the integrity of the medullary cavity. Pappas et al. '764 and Oh '841 each disclose a version of a cap that is implanted over the resected head of the femur. LaHaise '558 discloses a more complex means for attaching the ball to the resected head of a femur. One advantage to the surface replacement type of total hip replacement, is that much of the femur is left intact, so that if the surface replacement method fails, it may be replaced with an intramedullary canal prosthetic component.

Each of the above patents disclose a generally solid metal acetabular cup that is fixed, usually by screws, to a prepared surface of the hip bone. An insert, a layer of plastic or metal is frequently attached to the acetabular cup, the insert being sized to receive the ball portion of the prosthetic joint that is attached to the femur.

Each of these prostheses mentioned above, are installed during lengthy, invasive, major surgery that requires surgically opening the hip area for full exposure and direct access to the hip joint. During surgery the head of the femur must be removed from the acetabular cup, for resection of the femur head or complete removal of the femur head and neck. This surgery comes at a high cost as it is complex, requiring extensive surgical support staff and operating room equipment.

Not withstanding the existence of such prior art prosthetic components and methods for attachment to the human body, it remains clear that there is a need for prosthetic components that may be inserted into the human body without a major incision to gain direct access to the femur and hip bone.

SUMMARY OF THE INVENTION

The present invention relates to a prosthesis and method for implantation of that prosthesis within the human body as the replacement for a ball joint. Reduced or minimally invasive procedures secure a ball and shaft in the femur and a shell and cup in the acetabulum, for example. A segmented shell may comprise a plurality of separate segments which may be inserted minimally invasively into the patient and assembled and may be attached to the hip bone to form a socket.

One advantage is that a shell and cup and a shaft and ball may be implanted without fully opening or exposing the hip joint, directly.

An expandable drill bit is disclosed that is capable of removing a portion of the head of the femur and a thin portion of the outer layer of the acetabulum, for example, through a hole bored at an angle through the femur.

Another advantage is that the implant may be removed an a traditional implant may be inserted without causing damage or weakness that impairs the subsequent implant.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is an isometric view of a hip joint prosthesis;

FIG. 2 is a cross-sectional elevational view taken along line 2-2 of FIG. 1;

FIG. 3 is a top plan view of the segmented first shell of the prosthesis of FIG. 1;

FIG. 4 is an exploded cross-sectional view of FIG. 3 taken along line 4-4;

FIG. 4A is a detailed view illustrating the use of surgical thread;

FIG. 5 is an isometric view of a segmented shell with a key segment exploded;

FIG. 6 is a top plan view of a segmented second shell with a portion cutaway in cross-section to illustrate the relationship of the parts;

FIG. 7 is a cross-sectional elevational view taken along line 7-7 of FIG. 6.

FIG. 8 is a detailed front elevational view of one of the second group of parts of the segmented shell of this invention;

FIG. 9 is a cross-sectional view taken along line 9-9 of FIG. 8;

FIG. 10 is a side elevational view of one of the first group of parts of a segmented second shell;

FIG. 11 is a front elevational view of one of the first group of parts of a segmented second shell;

FIG. 12 is an isometric view of a shaft and cup;

FIG. 13 is a bottom plan view of a shaft;

FIG. 14 is an isometric view illustrating another example of a hip joint prosthesis;

FIG. 15 is an isometric view illustrating yet another example of a hip joint prosthesis;

FIG. 16 is a detailed view of a hip structure illustrating a placement of the guide wire into a hip bone;

FIG. 17 illustrates the placement of a protective hollow blunt guide and a hole bored through a neck and head of a femur and into a acetabulum of a hip bone;

FIG. 18 illustrates the placement of a sleeve into a hole bored through a femur;

FIG. 19 illustrates the insertion of an expandable bit into a sleeve;

FIG. 20 illustrates the extension and expansion of an expandable bit for removal of a portion of an acetabulum of a hip bone;

FIG. 21 illustrates the placement of a segmented shell and transportation of one of a plurality of segments through a sleeve guided by a secondary guide wire for placement onto a base and engagement with an adjacent segment;

FIG. 22 illustrates a placement of a segmented shell;

FIG. 23 illustrates a placement of a cup within a segmented shell of FIG. 22 and placement of a shaft into a hole bored through a femur;

FIG. 24 illustrates the placement of a segmented shell and a second segmented shell liner;

FIG. 25 illustrates the placement of a cup into the first and second segmented shells;

FIG. 26 illustrates a placement of a shaft into a hole through a femur;

FIG. 27 illustrates a placement of a segmented shell, a solid shell liner, a cup and a shaft;

FIG. 28 is a front elevational view an the expandable drill bit, illustrating the blades extended outwardly;

FIG. 29 is a detailed view of a first end of an expandable drill bit;

FIG. 30 is a detailed view of one of the blades of the cutter of FIG. 28;

FIG. 31 is a cross-sectional view taken along line 31-31 of FIG. 30;

FIG. 32 is a detailed top plan view of FIG. 28;

FIG. 33 is a detailed view of an attachment of the blades to an expandable drill bit;

FIG. 34 illustrates the attachment of the expandable drill bit to a drill motor for rotation of the expandable drill bit.

FIGS. 35A and 35B illustrate two examples of threaded shaft and ball assemblies;

FIG. 36 illustrates a sleeve fixed in a femur.

FIG. 37 illustrates a tapped bore hole in a femur;

FIGS. 38 and 39 illustrate alternative embodiments with and without a nail;

The examples described and drawings rendered are illustrative and are not to be read as limiting the scope of the invention as it is defined by the appended claims

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One example of a ball and shaft joint prosthesis is illustrated in FIGS. 1-13, in which shows the prosthesis 10 comprised of components capable of being used in a minimally invasive surgery. Based upon the teachings of this patent, a person of ordinary skill in the art is enabled to implant shoulder joint replacement, hip replacement, and other joint replacements with a less invasiveness, quicker recovery times, and fewer complications. FIG. 1 and FIG. 2 illustrate an example of a prosthetic joint comprising a socket implant 12 and a ball implant and shaft combination 114. The ball implant and shaft combination 114 is received by the socket implant 12 replacing the natural joint in a total hip replacement, for example.

For a total hip joint replacement, the acetabulum of the patient is prepared for receipt of the socket implant 12 and the femur is prepared for receipt of the ball implant and shaft combination 114. For a total shoulder joint replacement, the glenoid cavity of the scapula is prepared for receipt of a socket implant 12 and the humorous bone is prepared for receipt of the ball implant and shaft combination 114. In the case of a shoulder joint, the portion of the socket implant that is attached directly to bone may be solid or segmented, as the shoulder socket implant is much smaller than the hip socket implant, and the size of the humorous bone in comparison with the femur supports a larger access bore for passing a solid shoulder socket implant therethrough, although use of separate segments has the advantage of reducing the size of the borehole needed for inserting the socket 12.

For purposes of illustration, the hip joint will be used to illustrate the apparatus and method of implantation. A socket implant 12 comprises a first segmented shell 16 made of material, such as biocompatible metal, a second segmented shell 70 made from a material, such as a synthetic resin, and a cup 106, which may be made of any material having a hard surface capable of sustaining wear from a ball, such as a metal or cross-linked, high density polyethylene.

Another example is illustrated in FIGS. 14, 22, and 23, the socket implant 312 is comprised of a segmented metal first shell 316 and a cup 406 that is received by the first shell, the cup being made from metal or plastic. In order to use a single segmented shell and a cup, the patient receiving the prosthetic components 310 should have large bones such that the hole bored through the neck of the femur is large enough to allow thicker segments to pass therethrough and/or a greater or lesser number and thickness of segments may be used. Thus, the invention illustrated by the drawings may be modified to accommodate any bone size using minimally invasive surgery or other less invasive procedures.

Yet another example is illustrated in FIGS. 15 and 27, the socket implant 612 is comprised of a metal segmented first shell 616, which receives a second shell 670 that is constructed from a single piece of a material made of a ceramic, polymer or metal, which then receives a cup 706 of a synthetic resin, for example. A bore hole through the femur must have an opening large enough to accommodate the separate segments, second shell, ball and cup for use in minimally invasive surgery. Alternatively, a surgical technique may be used that is less invasive than standard procedures but not minimally invasive.

Now referring to the examples in FIGS. 1-13, the socket implant 12 comprises a segmented first shell 16, which may be mounted into a prepared hip acetabulum. Herein, the term “segmented shell” refers to a shell having separate segments, which may be inserted through a hole bored through a bone of a patient, which separate segments collectively form the concave surface of the shell when joined together. A first segmented shell 16 is comprised of a base 18 and a plurality of shell segments 20 a-j that engage one another to form the concave surface of the first segmented shell 16. Thus, a segment is defined as a separate piece and segmented is defined as something that is comprised of separate pieces. Any convenient number of segments can be used, provided each fits an opening cut into the patient; however, ten segments have been used for illustration purposes. The segmented first shell 16 is comprised of separate pieces or segments 20 a-j that are joined together to form the segmented first shell 16. As seen more clearly in FIG. 4, the base 18 has at least one ridge 22 that is formed in the top surface 24 of the base 18 proximal to the edge 26 of the base 18 and projects outwardly therefrom. In a preferred embodiment, the ridge 22 is continuous; however, this is unnecessary as long as a portion of the ridge 22 engages each segment 20 a-j. The base 18 has a hole 27 therethrough for receipt of a cannulated screw 29 for attachment of the base 18 to the hip bone. The bottom surface 28 of the base 18 is curved to fit the curvature of the prepared surface on the hip bone. A plurality of flexible secondary guide wires 30, one for each segment 20 a-j, are threadably mounted to the ridge 22 so that they are spaced apart from one another.

Each segment 20 a-j has an inner surface 32, having a concave curvature, an outer surface 34, having a convex curvature, a first longitudinal side 36, a second longitudinal side 38, a first end 40 and a second end 42. The outer surface 34 of each segment 20 may longitudinally and transversely arcuate, with a curvature that will mount to the curvature of the prepared surface of the hip. Each segment has a groove 44 formed in the outer surface 34 proximal the first end 40, as seen in FIG. 3 and FIG. 5, and a hole 46 that extends from the bottom of the groove to the inner surface 32. Some of the segments also may have a hole 48 that extends through the inner and outer surfaces of the segment between the first and second ends 40 and 42 of the segment. The holes 48 may receive surgical nails 50 therethrough. An example of a nail 50 is visible in FIG. 2, and is used to attach one of the segments 20 a-j to the prepared bone surface.

Also, the example illustrated in FIG. 2 shows how the cup 106 and shells 70, 16 may have an angled, exposed surface, such that the angle X provides for improved range of motion of the joint.

As seen most clearly in FIG. 5, all but two of the segments 20 a-j have male and female arms 52, 54 providing interlocking lands and grooves. The segments 20 a-g may be identical. Each male arm 52 extends outwardly from the first side 36 and then inwardly toward the longitudinal axis A of the shell 16. The arm 52 extends longitudinally from the second end 42 toward and proximal to the first end 40. A female second arm 54 extends outwardly from the second side 38 of each of the segments 20 a-j and then away from the longitudinal axis A of the shell 16 to form a groove 58. The groove 58 is sized and configured to receive the land 56 of the male arm 52 to form a land and groove interlocking connection. Wedge segment 20 i is the last segment to be attached to adjoining segments; therefore, wedge segment 20 i is inserted from the inside of the shell 12. The interior of the shell 12 has a smaller circumference than the exterior; and the wedge segment 20 i is wedge-shaped having an outer surface 34 narrower than the inner surface 32. The wedge shapes interlock the wedge segment 20 i with each of its neighbors. The adjacent sides of the two adjoining segments, 20 h and 20 j, are shaped to receive the wedge shape of the segment 20 i. In addition, wedge segment 20 i has two female arms 54 and the adjacent segment 20 h has two male arms 52 for interlocking with the adjacent segments.

For example, the segmented shell 16 may be constructed of titanium or cobalt chrome alloys. However, other materials that are resistant to wear, tough, and stiff may be suitable for the purpose.

The segmented shell 16 may be received by the surgeon in disassembled form for thorough disinfecting prior to assembly in the patient by the surgeon or may be packaged in a sterile pack in an assembly or pre-assembled state. During assembly of the segmented shell 16, each of the segments may be mounted on a respective secondary guide wire 30 and advanced along the wire until the groove 44 engages the ridge 22 on the base 18 as best represented by FIGS. 4, 44 and 21. The secondary guide wires 30 guide their corresponding segments to their proper position. These wires 30 are flexible enabling the segments 20 a-j to easily slide through the bore hole and into position during the surgery, which is described in greater detail herein. After assembly of the shell, the flexible wires 30 are removed from their holes 59, such as by screwing or unlashing and then removed. For example, holes may be bored through the base 18 to provide a through-hole 60, as illustrated in FIG. 4A. The first end 62 of a closed loop 64 of surgical thread, is passed through the hole 60 in the base 18 from the top downwardly. The second end 66 of the loop is then passed through the loop at the first end 62 to connect the thread to the base 18. The first end of the thread loop 66 is then passed through the hole 46 in one of the segments 20 a-j. Once the segments 20 a-j are in place, each of the plurality of loops of thread 64 are cut and the thread 64 is pulled from the base 18 through the hole 46 in the segments 20 a-j. Alternatively, the loop may be engaged on each of the segments 20 a-j and the passed through the base 18, which allows the segments 20 a-j to be pulled into place before cutting and removing the thread 64.

As shown in FIGS. 6-11, a second segmented shell 70 may be sized and configured such that it may be assembled within the first segmented shell 16. In this example, the second shell 70 is comprised of a plurality of parts. A total of 10 parts have been used for illustrative purposes, a first group of parts 72 a-e and a second group of parts 74 a-e; however, the number and size of the plurality of parts may be determined by the application. Each of the group of parts 72 a-e have a longitudinal axis B, a first end 76, a second end 78, a first face 80, a second face 82, a first side 84 and a second side 86. Each of the second group of parts 74 a-e have a longitudinal axis C, a first end 88, a second end 90, a first face 92, a second face 94, a first side 96, and a second side 98. The first face 80 and the second face 82 of the first group of parts and the first face 92 and the second face 94 of the second group of parts are each longitudinally and transversely arcuate. The first ends 76 and 88 of each part of the first and second group of parts is linked to one another, preferably by surgical thread passing through the hole 100 in the parts 72 a-e and the hole 102 in the parts 74 a-e, as seen in FIGS. 10 and 8 respectively.

A portion of the sides 84 and 86 of each of the parts 72 a-d, that is proximal the outer rim 104 of the shell 70 extend radially toward the central axis D. When the parts 74 a-e are being assembled to form the cup shaped shell 70, the last part 72 e is tapered outwardly such that the sides 84 and 86 are angled toward one another. The horizontal arc of the second face 82 is smaller than the horizontal arc of the first face 80. To ensure that all sides fit tightly, the sides of the adjacent parts, 72 a and 72 d, are tapered inwardly to match the taper of part 72 e such that the sides fit tightly when the parts are fully assembled.

The sides 84 and 86 of each of the parts 74 a-d are tapered outwardly so that the first face 92 is much larger than the second face 94. The transverse dimensions of the second face 94 of the parts 74 a-d vary along the longitudinal axis C between the first end 88 and the second end 90. The horizontal width of the second face 94 varies from a sharp edge (where the cross-section of the part would be triangular) proximal the first end 88 to a greater cross-section as seen in FIG. 9 and then narrows proximal the second end 90 to a generally triangular cross-section. As each of the parts 74 a-e is inserted between adjoining parts 72 a-e, as illustrated by FIG. 6, the curvatures of the adjacent sides match one another to make a tight friction fit, for example.

The second shell 70 may be constructed from a synthetic resin or a suitable metal. In one example, the second shell is formed in place by injecting a polymer resin, which hardens when cured, as a alternative to a second segmented shell. The second segmented shell 70 may be made using the same material used for the first segmented shell. The second segmented shell may be comprised of interlocking segments and a base similar to the first segmented shell 16, as an alternative to the structure illustrated in FIGS. 6-11. The primary difference is that the second shell is smaller than the first segmented shell 16. However, the second shell may be made of a suitable synthetic resin or metal of a different composition than the first shell 16. The second shell when made of metal may be attached to the first segmented shell 16 by snap rings, for example. A polymer resin shell may be joined to the first segmented shell 16 by heat and pressure or by snap rings. In one example, the inner surface 32 of the first shell 16 may be roughened or otherwise prepared to make a tight bond to a polymer resin shell.

As shown in FIG. 2, a cup 106 is inserted within the second shell 70. In one example, the cup 106 is made of a solid titanium or cobalt chrome alloy with a highly polished interior surface 108. The cup 106 is preferably a metal when the second shell 70 is constructed from a synthetic resin, for example. The cup 106 may be constructed from a synthetic resin, such as a cross-linked high density polyethylene. When comprised of a synthetic resin, the cup 106 may be formed so that when the cup 106 receives the ball 110, the edge 112 of the cup 106 extends beyond the equator of the ball 110 so that the cup 106 snaps onto and is retained on the ball 110 for easy insertion into the socket implant 12.

The cup 106 may be retained in the second shell 70 by a snap ring, press fit, adhesive bond or other retainer. The shells and the cup may be attached to one another in a number of different ways. Snap rings or drop rings operate simply. Snap ring or drop ring 145, as seen in FIG. 25, is inserted within a groove 146, as seen in FIG. 5 that matches a second groove 148 in the second shell 70, as seen in FIG. 7. When the second shell 70 is pressed into place, the snap ring 145 is expands into the groove 146 locking the two shells together. Snap ring 188 in groove 190 may engage the groove 182 in the cup 106. When a plastic cup or plastic segmented shell is to be attached to an inner metal shell, heat (approximately 400 degrees) and pressure can be applied to the plastic cup or shell so that it bonds to a roughened surface of the metal shell. In other embodiments, it is possible to use well-known biologically tolerable bonding resins to adhesively bond a shell or cup within a shell.

A ball implant and shaft combination 114, as shown in FIGS. 12 and 13, has a ball 110 opposite of angled end 118. A shaft body 120 and a neck 122 are fixed to the ball 110. The ball 110 may be shaped spherically and may be comprised of highly polished titanium or cobalt chrome, for example. The neck 122 and the body 120 may be comprised of a steel alloy and the body 120 may be coated with a bone growth medium, such as any one of the well-known ingrowth surfaces for encouraging the growth of the bone to that surface for bonding the body 120 to an adjacent bone, such as a femur. The shaft 114 is cannulated, having at least one channel 124 extending from an open first end 126 at a bottom surface 128 of the opposite end 118 of the shaft body 120 to an open second end 130 on a surface of the neck 122 disposed between the shaft body 120 and the ball 110. The channel 124 may be used to flush and suction the site of the hip joint to remove debris that may have collected in the joint during surgery or due to the wear of the head 110 or cup 106 and may also be used to insert antibiotics to fight infection. A bag constructed from a formulation of silicon and rubber (or other material that is non toxic to the surrounding tissues) may be implanted in the patient's leg adjacent the second end 118 of the shaft 114 and attached by a ring mechanism or other fastener or adhesive to the shaft 114 so that it is in fluid flow communication with the tube or tubes 124. The bag will collect debris created by the prosthetic joint to prevent the debris from a toxic interaction with the surrounding tissue. An open second tube 132 also extends inwardly from the bottom of the shaft 114 to a closed second end 133 proximal the neck 122. The second tube 132 has an open first end 134 that is threaded for attachment of a syringe through which a cement may be injected into the tube 132 to bond the body 120 to the surrounding bone. The threads on the second tube 132 may also be used for attachment of an extraction tool (not shown) to remove the shaft 114, if the implant is temporary or damaged. A secondary tube 136 may be connected at one point along the second tube 132 and may be in fluid flow communication such that a fluid may be distributed to a surface of the shaft body 120. For example, the secondary tube 136 extends through the side wall 137 and additional tubes 136, 136′ provide the channels to the other surfaces the body 120 such that cement may be distributed between the body 120 and the surrounding bone. For an even distribution of the cement around the body 120, a plurality of secondary tubes 136, 136′, as illustrated in FIG. 12 for example, interconnect with the second tube 132. For example, in FIGS. 12 and 13, two channels 124 are formed in the shaft body 12 extending from an opening 124 in the bottom surface 128 to an opening in the neck 130. The body 120 has holes 184 for screws 138 for attachment to the bone of the femur, as shown in FIGS. 2 and 12, for example. As shown in FIG. 2, the screws may have expandable ends 188 to prevent loosening and provide a fixed attachment to the bone, which is particularly desirable if the bone is cancellous or otherwise weak.

A groove 140 may be formed in the side wall 137 adjacent the bottom 128 to receive a U-shaped shield 142 that has a pair of legs 144 that extend outwardly from the groove. This shield is curved to fit the body 120 so that the legs will engage the comer of the cortical bone along the superior portion of the hole through the femur to reduce the risk that the bone will fail due to stresses applied by the body 120 of the shaft 114.

When the patient's bone structure is large enough or the prosthesis small enough, FIGS. 14, 22 and 23 illustrate another example of the invention. Segments 420 a-j (not all of which are shown), may be inserted and assembled to form a single, segmented shell 416 and cup 406 or a dual shell 616, 670 and cup 706, as illustrated in FIGS. 14 and 15, respectively. The socket implant 310 comprises the segments 420 a-j that are assembled and joined to one another to form the first shell 316, and a cup 406 is inserted. The cup 406 is sized and configured to be received into the interior cavity of the first shell 416. The cup 406 is held in the first shell 416 by a snap ring 445, for example, as illustrated in FIG. 23. The cup 406 may be constructed of a metal or a polymer resin. Attachment by a snap ring may be done whether the cup 406 is metal or resin; however a resin may be able to adhere to the surface of the shell 316 without the need for a snap ring, for example. The shaft 414 may be similar in size and shape as shaft 114, illustrated in FIGS. 1-13. The cup 406 is sized to receive a ball 410.

A compound shell 610, illustrated in FIGS. 15 and 27, maybe similar in structure and assembly to the previous examples, except a solid second shell or shell liner 670 is inserted between the first segmented shell 616 and the cup 706. As shown in FIG. 15, the shell liner 670 is formed as a single piece, which is not segmented. The single piece shell liner 670 may have the same general shape as the segmented shell 70. The maximum diameter of the shell liner 670, during installation, must be less than the minimum diameter of the channel through which it is inserted during surgery to assemble the compound shell 610 (not shown). The second shell 670 may be made of metal such as a titanium or cobalt chrome alloy. A polymer resin cup 706 may be inserted in the second shell 670 and may be attached by snap rings or heat and pressure, as previously discussed. The combined ball and shaft 714 may be similar in structure to the previously described examples.

An advantage of this apparatus and method is that the surgery is much less invasive, takes much less time, requires much less surgical support in the operating room, and is much less expensive. In addition, if these components were to fail, a full hip joint replacement, done in accordance with current practice, is still available, as more than enough of the femur remains and only a small portion of the acetabulum was removed. The steps for implantation of these prosthetic components are discussed below. Thus, the examples are suitable for use as a temporary prosthetic device, for example.

The patient is suitably prepared for surgery in accordance with known practice. In an example of a method of using a prosthetic device in a minimally invasive surgery, the patient's body is aligned so that a longitudinal axis D extending from and neck and head of the femur of the patient passes through the geometric center of the acetabulum of the patient, such as illustrated in FIG. 16. The patient's body is placed in skin traction to prevent movement during surgery. A one inch incision is made to expose the femur at the point at which the longitudinal axis exits the femur, just under the greater trochanter. As seen in FIG. 16, a hollow and blunt guide 150 is inserted up against the bone such that its axis is coincident with the longitudinal axis D. A 2 mm smooth guide wire 152 with a threaded tip, as shown in the enlarged view of FIG. 16, is passed through the guide 150 and along the longitudinal axis D so that the guide wire 152 is aligned with and driven through the geometric center of the femoral head, across the joint and into the geometric center of the acetabulum 156. A cannulated drill bit is mounted over the guide wire 152 and a hole is bored through the femur, the neck of the femur, the head, across the joint and 2 mm into the acetabulum. A larger blunt guide 150′ is placed over the smaller blunt guide 150, which is then removed. A second cannulated drill bit may be inserted in the drill to bring the hole 158 to the proper diameter, as shown in FIG. 17, for example. The size of the hole 158 will largely be determined by the size of the patient's femoral neck, in the case of a femoral joint replacement. However, other factors may include the age and athletic activity of the patient and whether the prosthesis is permanent or merely temporary.

The hole 158 for example, does not remove the interior of the subchondral plate. To ensure that the bore does not extend beyond the planned depth into the acetabulum, such as 2 mm, the cannulated drill bit and drill ride the guide wire 152 to a stop 153, as seen in FIG. 31. FIG. 34 demonstrates use of the drill 200 with an expandable drill bit 166, which is used for forming the cavity in the acetabular. However, a cannulated surgical drill bit may be inserted in the drill motor 200 to bore hole 158. A guide wire 152 may be received within a tube in the drill motor 200 that has a predetermined length. The guide wire 152 is sized so that when the end of the guide wire 152 reaches the end of the tube, or stop 153, the proper length of the hole 158 has been reached. In this way, the depth of a hole may be accurately predetermined.

As seen in FIG. 18, sleeve 160, having the same exterior diameter of the hole 158, which is bored in the femur, is now placed around the wire and inserted through the blunt guide 150′ and into the femur until the first end 162 of the sleeve 160 lies proximal the neck 164. The blunt guide 150′ may be removed. An expandable drill bit 166, as illustrated in FIG. 28, for example, is mounted on the guide wire 152 and inserted through the sleeve 160. As shown in FIG. 19, the expandable bit is capable of holding to less than the inner diameter of the sleeve 60. This drill bit 166 has multiple blades 168 that can be expanded so that the outer cutting edges 170 of the blades 168, when fully expanded, match the shape of the outer surface of the segmented shell 16. As seen in FIG. 20, once the blades have exited the sleeve 160 they may begin cutting away the head of the femur 154. The expandable drill bit 166 is advanced until the drill blades 168 remove approximately 2 mm of the acetabulum 156, for example. The expandable drill bit 166 includes a mechanism for measuring the distance that has been bored. Once the predetermined distance for movement of the drill along the guide wire has been reached the expandable drill bit 166 is drawn inwardly so that the inner cutting edge 172 of the blades 168 can remove additional portions of the head 154 such that adequate clearance for full movement of the joint is accomplished. In one example, a portion of the femoral head and neck remains, in order to provide a structure for a later procedure greater stability to the shaft body. The expandable drill bit 166 is advanced again until the blades 168 can be closed and then the expandable drill bit 166 is retracted through the sleeve 160. The drilling operation may be controlled through fluoroscopy. While drilling is being accomplished a flushing fluid may be injected into the site through a tube 174 in the expandable drill bit 166 and suction may be applied to the central portion of expandable drill bit 176 for removal of debris.

Once debris is removed, a socket implant may be attached to the hipbone of the patient. A ball 110 is sized to be inserted through the hole 158, and the acetabulum site on the hipbone has been enlarged by the expandable drill bit 166. Therefore, the cup 106 is sized to receive the ball and the cup 106 is supported by one or more shells attached to the hip bone. The larger the cross-section of the neck of the femur the larger the hole 158 that can be bored through the femur, without damage to the subchondral plate, through which the socket implant is passed. The surgeon, based upon measurements of the patient's bone structure, may determine the particular size and structure of the socket implant.

In one example, prosthetic component 10 comprises a segmented first shell 16, a segmented second shell 70, a cup 106, and a shaft 114 of FIG. 1. In FIG. 14, a second example comprises a socket implant 312 comprised of a metallic segmented first shell 316 and a metallic or polymer resin cup 406. A third example comprises a segmented first shell 616, a single piece, metal second shell 670, and a cup 706, such as illustrated by FIG. 15. The fewer parts needed to complete the socket implant, the easier the placement and the more quickly the operation can proceed, which is better for the surgeon and the patient.

In one example, the surgeon selects the example using two segmented shells. The next step is to implant the first segmented shell 16. The base 18 having the cannulated screw 178 inserted therein, is mounted on the guide wire 152 so that the cannulated screw and base 18 passes along the guide wire 152 and is therefore centered in the acetabulum . With the base 18 centered by the guide wire the self-tapping cannulated screw 29 is driven into the bone of the acetabulum of the patient by a cannulated screw driver. The ends of the plurality of flexible wires 30 attached to the base 18 may extend outwardly through and beyond the sleeve 160. First a segment having two male arms 52, which in FIG. 5 is segment 20 h, is inserted. A flexible secondary guide wire 30 is received through the hole 46 in the first segment 20 h so that the segment 28 may slide downwardly on the guide wire until the groove 44 on the first segment 20 h engages the ridge 22 of the base 18, as seen in FIGS. 4 and 5. Once the segment is seated in place a surgical nail 50 is driven through the hole 48 in the segment 20 h to attach the segment 20 h to the hip bone. As seen in FIG. 21, some of the segments are already in place and one segment is shown mounted on one of the flexible secondary guide wires 30, such that the flexible secondary guide wire 30 passes through the hole 46 in the segment. Thus, the guide wire 30 directs and aligns the segment 20 c to the proper location.

After positioning of segment 20 h, segment 20 g is then mounted on its corresponding flexible secondary guide wire 30 and installed so that the female land 56 engages the groove 58 of the segment 20 h. The next segments are inserted in the same manner until the next to last segment 20 j is in place. The last segment, segment 20 i, has two female arms 54 that engage with the male arms 52 on segments 20 h and 20 j interlocking the segments 20 a-j of the segmented first shell 16 in place. In addition, as discussed previously, the segment 20 i is tapered so that it may be inserted in place from the inside of the first shell 16. A surgical nail is driven through the hole 48 of segment 20 i to secure the segments. As seen in FIG. 21, surgical nails may be driven in more of the holes 48 in the segments 20 a-j if the surgeon believes it is desirable. The location of the nails will be determined by the surgeon based upon the thickness and density of the bone in the adjacent area; however, nails are often driven into the superior, posterior superior or the straight posterior portions of the hipbone, as needed. If nails are driven in segments 20 h and 20 i, then these segments should be located adjacent to bone that is thick and dense.

FIG. 21 illustrates the method for placement of a segment, and it illustrates the implantation of the first segmented shell 16, with the plug 180 screwed into place. An asymmetric surface contour 3 is shown that provides a greater angle ({acute over (α)} in FIG. 2) in one direction than another, improving range of motion in at least one direction requiring greater range of motion. As discussed previously, a loop of surgical thread 64, as shown in FIG. 4A, may be used as a guide wire or may be used in addition to a guide wire to deliver the segments to their proper location.

A second segmented shell 70 is likewise positioned within the first shell 16. FIG. 24 schematically illustrates a second segmented second shell 70 inserted within the first segmented shell 16.

As shown in FIGS. 25 and 26, the second shell 70 lies between the first shell 16 and the cup 106. As shown in FIGS. 6-11, the segmented second shell 70 comprises a plurality of parts, a first group of parts 72 a-e and a second group of parts 74 a-e. The first group of parts 74 a-e each have a hole 100 therethrough and the second group of parts each have a hole 102 therethrough. The parts are alternatingly strung on a surgical thread that is passed through the holes 100 and 102 and tied in a loop. The parts are then inserted through the sleeve 160 so that the guide wire 152 passes through the loop of surgical thread. The surgeon places the first group of parts within the first shell 16 beginning with part 72 a and ending with part 72 e. The last part 72 e is tapered to provide a friction fit between part 72 d and 72 a, forcing the other parts into position forming a cup-shaped shell. The second group of parts 74 a through 74 e, which are held in proper orientation by the surgical thread, are then each inserted between the adjacent pair of the first group of parts 72 a-72 e. As seen in FIG. 6 and 9, the second group of parts are tapered and may be readily placed in position and then firmly pushed into place. The parts 72 a-e and 74 a-e are then placed under pressure and high temperature (approximately 400 degrees), thereby bonding the plurality of parts 72 a-e and 74 a-e to one another and to the first shell 16.

Alternatively, if the second segmented shell 70 has a similar interlocking arm structure as the first segmented shell 16, the segments are inserted through the sleeve 160 and assembled in the first segmented shell 16 in the same manner that the first segmented shell 16 was inserted through the sleeve 160 and assembled in the acetabulum. Of course, the first and last segments of the second shell may be attached or bound to the first shell 16 by heat and pressure or using a snap ring, for example.

The next step is to insert a cup 106 through the sleeve 160 and fit it to the second shell 70 such as by using a snap ring 188 that is inserted in the groove 190 in the parts 72 a-e. As the cup is pushed into the interior cavity of the first shell 16, the exterior sides of the cup 106 engage the snap ring 188 pushing it into the groove 190 until the groove 190 aligns with the groove 182 in the cup 106, at which time the snap ring 188 expands outwardly and engages the groove 182 locking the cup 106 within the second shell 70, as seen in FIG. 25. If the cup is made from a polymer resin it may be installed on the ball 110 prior to the shaft 114 being inserted in the hole 158. The shaft 114 may be pressed into the hole in the femur, and the ball 110 with the cup attached seats the cup 106 in the second shell 70. The cup 106 may be locked by a snap ring in the second shell 70, for example. As discussed previously the plastic cup 106 snaps on the ball as the edge 112 of the synthetic resin cup curves around the ball beyond its equator.

The sleeve 160 may be removed from the hole 158 prior to inserting the ball and shaft assembly 114. As illustrated in FIG. 23, the shaft 114 may be implanted in the femur of the patient. The shaft 114 may be driven into the hole 158 so that the ball 110 is seated within the cup 106 for free movement between the ball and the cup. As the shaft 114 is inserted, a shield 142 is placed around the superior portion of the hole 158 and is held in place by the groove 140 formed in the body 120. To stabilize the shaft 114 during the healing process, a surgical screw 138 may be inserted through a hole 184 in the shaft 114. The surgical screw 138 may have a first end 186 that is capable of expanding after it has entered into the femur to more tightly hold the shaft 114 in place. This may be especially desirable if the bones are soft. Additional screws may also be inserted through other holes. If the patient has very soft bones, such as due to osteoporosis, a syringe may be threadably attached to the tube 132 and a fluid cement may be forced into a channel the tube 132 and out secondary channels 136 and 136′ between the sidewall of the shaft body 120 and femoral bone. Also, if the bones are soft a shaft with a larger diameter may be driven into the femur for a better bond, as long as the subchondral plate of the bone, the hard exterior layer of bone, is drilled to the size of the shaft.

The shaft 114 may be cannulated by at least one tube 124 which permits flushing and suctioning of the hip joint site prior to closing the incision and at a later date, if infection or other difficulties occur. At this time, the surgeon may attach and implant a drainage bag constructed from a formulation of silicon and rubber, not shown, to catch any drainage after the incision is closed. The incision may now be closed. At a later date it may be necessary to remove and replace the drainage bag.

If the surgeon determines, through measurements of the patient's bone structure and other factors such as age, athleticism and purpose of the prosthetic, that the bone structure can support an alternative prosthetic component 310, the surgeon may select a prosthetic component 310 having fewer parts, reducing the complexity of the procedure. The steps for implantation of the prosthetic component 310 will largely be the same as the previous procedure, except a second segment shell is not inserted. All the steps leading up to and for installing the first segment 316 are the same as described above.

The next step is to insert a cup 406 through the sleeve 460, the cup being sized and configured to be received into the interior cavity of the segmented shell 316. The steps for attachment of the cup 406 to the shell 316, by snap ring 445 are the same as discussed above for attaching the cup 106 to the second shell 70, except the snap ring 445 is positioned in the segmented shell 316.

FIG. 23 shows the completed installation of the prosthetic component 310, with similar steps for inserting and fixing the shaft body 414 in the hole 458.

The surgeon may determine that a second shell or shell liner may be formed as a single piece, as shown in FIGS. 15 and 27, that will pass through the hole 458 as shown in FIG. 27. Therefore, he may select the prosthetic component 610, as shown in FIG. 15. The steps for installation of the prosthetic component 610 change only to the extent that the shell liner 6 f 70 (or second shell) is inserted and fixed in place prior to insertion of the cup 706. The steps for installing the single piece second shell 670 will require that the shell 670 be sized to pass through the hole 458 for minimally invasive surgery. However, a reduced invasiveness may be obtained by inserting a shell or a cup through a separate incision providing limited access to the joint of a patient for introducing the shell or cup at the site of the joint, for example. The shell 670 will then be inserted into the interior of the first shell 616 and attached thereto by a snap ring, or otherwise, in the same manner that the cup 106 was attached to the second shell 70, as discussed above. In one example, a solid (single piece) shell liner 616, is made of polymer resin, which may be bonded to the first shell by pressure and temperature. The shaft 714 is installed by the same steps used to install the shaft 114, as described above. FIG. 27 shows the completed installation of a prosthetic component, with the ball 710 inserted into the cup 706. In the example illustrated in FIG. 27 the shaft body 714 is symmetric, providing a greater range of motion of the joint in one direction than in another.

The surgeon may determine that none of the segmented acetabulums will be appropriate and may insert any of the well known acetabulums directly into the prepared hip socket. These acetabulums may be selected that have a cup sized to receive the ball 110, 710. The surgeon may determine that minimally invasive surgery is not preferred, but the surgeon may use a reduced invasiveness, as previously discussed. For example, the femoral head may be partially resected and the neck and head portion may be extended through an incision without fully and directly exposing the joint. FIGS. 35A and 35B illustrate two examples of ball and shaft assemblies 3510 and 3520. These assemblies may be inserted by tapping and/or threading the femur, such as shown in FIG. 37, after removing a portion of the femoral head.

In FIGS. 35A and 35B, the shaft and ball assembly 3510, 3520 may be inserted directly into a hole 3700 bored int the femur 3701. Threads 3514 may be self-tapping or may mate with tapped threads. Holes 3516 may be used to inject adhesive and bone growth stimulant between threads 3514, using one or more channels 3518 extending from a neck region. An exit hole 3519 may be used for injecting cement at the base of the shaft 3512. In FIG. 35B, the base 3529 is dome-shaped. The dome-shaped base 3529 may be made of flexible material, such as silicone rubber or may be rigid.

In FIG. 36 a tube 3530 is inserted in a hole bored into the femur by screwing the self tapping threads 3534 into the hole. Additional holes 3536 are located between threads for injection of a biocompatible adhesive and bone growth promoting material between the tube 3530 and the bone. A ball and shaft assembly may be mechanically or adhesively fixed in the sleeve formed by the tube 3530.

FIGS. 28-33 disclose a preferred embodiment of an expandable drill bit 166. In FIG. 28, the expandable drill bit is shown extending through the sleeve 160. The expandable drill bit 166 comprises a plurality of blades 168, a body 190 and a central hollow shaft 192 that includes a plurality of supports 194. As seen in FIG. 29, the top plan view of the apparatus of FIG. 32, a support 194 is attached to the first end 196 of the shaft 192, and at least one additional support 194, as seen in FIG. 28, is spaced along the length of the shaft 192. The support 194 has a hole 193 therethrough for receiving the guide wire 152 therethrough and a plurality of holes 195 that permits water to pass therethrough for flushing the site. The second end 198 of the shaft 192 is insertable in a drill motor 200, as shown in FIG. 34, for rotation of the shaft 192 and the blades 168 of the expandable drill bit 166.

Annular plate 202, as shown in FIGS. 32 and 33, is attached to the first end 196 of the shaft 192 for attachment of the blades 168 to the shaft 192. Each blade 168 has one end of a stiff wire 204 attached to a hole 206 in a respective blade of the plurality of blades 168. The other end of the stiff wire 204 is attached to the end support plate 208 which is mounted to the first end 210 of the body 190. The body 190 is slideably mounted on the shaft 192 for expansion and retraction of the blades 168. A flexible wire 212 is passed through the holes 214 of the blades 168 as a safety measure to prevent the blades from expanding beyond a predetermined arc with a radius matching the finished radius of the acetabulum.

The hollow shaft 192 provides a means for delivering a flushing fluid to the cutting site through the port 216, which is connected to a pressurized water supply, (not shown). The port 216 is connected to a fixed annular ring 218, as seen in detail FIG. 29, that is sealingly attached to an annular cavity 217 that extends about the body 190 so that the body 190 may rotate inside the ring and maintain the port 216 in fluid flow communication with annular cavity 217 and the hollow shaft 192. A water source is attached to the port 216 by any well known means. Suction may be applied to port 220 by any well known suction device (not shown). Port 220 is connected in fluid flow communication with the interior of the body 190 through a fixed annular ring 224, for rotation of the body 190 therein, and an annular cavity 222. This permits suctioning the flush water and debris from the hip joint site through the plurality of holds 226 through the end plate 208 and the hollow body 190.

As the body 190 is free to slide longitudinally on the shaft 192, it is also free to rotate about the longitudinal axis of the shaft 192. During a cutting operation the body 190 must rotate with the shaft 192 to maintain the wires 204 in proper orientation. Therefore, thumbscrew 228 is tightened to rotate the body 190 with the shaft 192 and is loosened when adjustments are made to the angle of the blades 168. For adjustments to be made to the blades 168, the drilling must be stopped, the thumb screw 228 loosened, and the body moved along the shaft 192.

As discussed previously, each blade 168 has an outer cutting edge 170 and an inner cutting edge 172. The outer cutting edge is used primarily for cutting through the head of the femur and cutting the acetabulum to its predetermined curvature. The inner cutting edge is used to further trim the neck and head of the femur to ensure adequate clearance for free movement of the prosthetic joint.

In FIGS. 38 and 39, two alternative examples are shown for fixing a shaft and ball assembly 3512 in a femur 3701. In FIG. 38, threads and an adhesive cement injected between the threads are used to fix the femoral implant, while in FIG. 39, a nail 3910 is inserted through a hole 3590 and one or more pins, nails or screws 3912 may be used to further secure the nail 3910, such as gamma nail. This procedure is less invasive than traditional total joint replacement. The addition of a nail 3910 may be desirable when there are fractures or substantial bone loss, for example.

The foregoing examples are not limiting and should not be used to limit the claims. Instead, the claims should be read in light of the specification as a whole according to their plain meaning to a person of ordinary skill in the field. 

1. A joint prosthesis, comprising: An implant comprising a neck joining a ball to a body of the implant and a flange extending spirally from an outer cylindrical surface of the body such that the spiral flange extends into a bone and increases the contact area between the implant and the bone, when the implant is threaded into the bone, wherein the implant has a cylindrical axis extending along the cylindrical axis of the outer cylindrical surface and through the ball of the implant.
 2. The joint prosthesis of claim 1, wherein a plurality of channels are formed in the implant such that holes on the outer cylindrical surface are in the fluid communication with a channel capable of delivering a cement between the outer cylindrical surface and the bone.
 3. The joint prosthesis of claim 1, wherein an end of the implant opposite of the ball is dome-shaped.
 4. The joint prosthesis of claim 1, further comprising a socket capable of receiving the ball of the implant in a cup of the socket.
 5. The joint prosthesis of claim 4, wherein the socket comprises a shell and the cup is fixedly supported by the shell, and the shell is capable of being fixed to a bone structure in the body of a patient undergoing joint replacement surgery without fully opening and exposing the hip joint.
 6. The joint prosthesis of claim 5, wherein the shell is comprised of a plurality of separate segments, each having a concave arcuate surface such that the concave surface for supporting the cup is assembled by joining the concave arcuate surfaces of each of the plurality of separate segments.
 7. The joint prosthesis of claim 6, wherein a plurality of the plurality of separate segments have an interlocking member capable of interlocking with an interlocking member of a neighboring interlocking member, when assembled to form the shell.
 8. The joint prosthesis of claim 7, wherein a key segment has a wedge and two femal interlocking members each of the two femal interlocking members being capable of engaging a male interlocking member of each of its two nearest neighboring segments, and the wedge locks the plurality of segments together.
 9. The joint prosthesis of claim 6, further comprising a base having at least one ridge extending from the base and a fastener for fastening the base to the bone structure in the body of the patient, and a plurality of the plurality of segments each have a groove capable of pivotally engaging the at least one ridge of the base during assembly of the shell.
 10. The joint prosthesis of claim 5, wherein the ball and the shell, when assembled to form the socket, define an outwardly facing surface opposite of a surface of the socket facing the bone structure, and the outwardly facing surface is nonplanar.
 11. The joint prosthesis of claim 10, wherein the nonplanar outwardly facing surface is arranged such that the implant has a range of motion greater than the range of motion of an implant having a planar outwardly facing surface.
 12. A joint prosthesis comprising: a segmented shell having an interior surface, a second shell fixedly retained within said interior surface of said segmented shell; a cup, said cup having an exterior surface and an interior surface, said exterior surface of said cup being sized and configured to be fixedly retained in said second shell, and a shaft having a first end and a second end, said first end of said shaft having a ball formed thereon that is received by said cup for movement therein, wherein the segmented shell comprises: a base; and a plurality of segments, each segment, of said plurality of segments, having a first end, a second end and a pair of opposing sides extending therebetween, each first end of each said segment, of said plurality of segments, pivotally engaging said base such that each side of each said segment, of said plurality of segments, is adjacent one of said sides of another one of said plurality of segments, each segment, of said plurality of segments, being longitudinally and transversely arcuate, such that when each segment, of said plurality of segments, is aligned with adjacent segments thereto, said plurality of segments form a cup-shaped shell.
 13. A joint prosthesis as in claim 12, wherein said base has a ridge formed thereon and each segment, of said plurality of segments, has a groove proximal one end of each segment capable of pivotally engaging said base.
 14. A joint prosthesis as in claim 12, wherein said second shell is formed from a polymer and the polymer adheres to the surface of the segmented shell.
 15. A joint prosthesis comprising: a first segmented shell having an interior surface comprised of a plurality of separate segments; a second segmented shell supported by the first segmented shell, the second shell being comprised of another plurality of separate segments; a cup, the cup having an exterior surface and an interior surface, said exterior surface of said cup being sized and configured to be fixedly retained in the second segmented shell, and a shaft having a first end and a second end, the first end of the shaft having a ball that is received by the cup.
 16. A joint prosthesis as in claim 15, wherein the shaft has at least one channel passing through a body of the shaft such that the at least one channel extends from an opening in the second end of the shaft through the body to an opening in the neck.
 17. A joint prosthesis as in claim 16, wherein the shaft has a plurality of channels, and at least two channels are in fluid flow communication.
 18. A joint prosthesis as in claim 15, wherein the shaft has at least one longitudinally extending side wall and a groove formed in said side wall proximal the second end of the shaft, and a U-shaped shield has a bottom and a pair of legs extending outwardly from the bottom, the bottom being received in the groove such that the pair of legs extend outwardly form the shaft.
 19. A joint prosthesis as in claim 15, wherein the shaft has a flange extending spirally from an outer cylindrical surface of the shaft.
 20. A joint prosthesis as in claim 12, wherein the shaft has a flange extending spirally from an outer cylindrical surface of the shaft. 